FIG. 1 shows a structure of a conventional electric field sensor head 101 implemented by a waveguide-type element. The electric field sensor head 101 comprises a substrate 102 made of a lithium niobate crystal which is cut out perpendicularly to a c axis, an incident optical waveguide 103, phase-shift optical waveguides 104 and 105 branched from the incident optical waveguide 103, and an outgoing optical waveguide 106 into which the phase-shift optical waveguides 104 and 105 are joined and coupled. The incident, the phase-shift, and the outgoing optical waveguides are formed by diffusion of titanium on the substrate 102. An incident optical fiber 107 is connected to an incident end of the incident optical waveguide 103 while an outgoing optical fiber 108 is connected to an outgoing end of the outgoing optical waveguide 106.
A pair of electrodes 109 are formed on the phase-shift optical waveguides 104 and 105. These electrodes 109 are connected to rod antennas 110. Referring to FIG. 1, an incident light beam 111 is incident through the incident optical filer 107 to the incident optical waveguide 103 and then divided in energy into the phase-shift optical waveguides 104 and 105. When an electric field is applied, electric voltages are applied to the electrodes 109 by the rod antennas 110. In the phase-shift optical waveguides 104 and 105, electric field components which have directions opposite to each other in depth directions are produced. As a consequence, a variation in refractive index is caused by an electrooptical effect so that a phase difference corresponding to the magnitude of the applied electric field is produced between light waves propagating through the phase-shift optical waveguides 104 and 105. When the light waves are combined and coupled at the outgoing optical waveguide 106, a light intensity is varied due to interference. Specifically, the intensity of an outgoing light beam 112 emitted through the outgoing optical fiber 108 is varied in response to the intensity of the applied electric field. By measuring the variation of the light intensity, it is possible to detect the intensity of the applied electric field.
FIG. 2 shows an electric field sensor using the above-described conventional electric field sensor head 101 illustrated in FIG. 1. The incident optical fiber 107 of the electric field sensor head 101 in FIG. 1 is connected through a transmission optical fiber 113 to a light source 114 while the outgoing optical fiber 108 is connected through a reception optical fiber 115 to an optical detector 116. Although not shown in FIG. 2, a detected electric signal from the optical detector 116 is connected to an ordinary measuring unit such as a voltmeter, an ammeter, and a spectrum analyzer.
However, it is difficult to use the conventional electric field sensor of the type described in detection of a high electric field because, upon detection of the high electric field, the electrodes are easily damaged due to discharge of electric charge induced by the voltages applied to the two electrodes spaced at an interval as small as several microns to several tens of microns.
The conventional electric field sensor of the type described has another problem that a detection sensitivity is not so good. This is because electrode capacitance is generally increased in response to the length of the electrode.
It is an object of this invention to provide an electric field sensor adapted for use in detection of a high electric field.
It is another object of this invention to provide an electric field sensor having an excellent detection sensitivity.